dt-binding: iio: add NPCM ADC documentation

Nuvoton NPCM Analog to Digital Converter (ADC)

The NPCM ADC is a 10-bit converter for eight channel inputs.

Required properties:

- compatible: "nuvoton,npcm750-adc" for the NPCM7XX BMC.

- reg: specifies physical base address and size of the registers.

- interrupts: Contain the ADC interrupt with flags for falling edge.

Optional properties:

- clocks: phandle of ADC reference clock, in case the clock is not

		  added the ADC will use the default ADC sample rate.

- vref-supply: The regulator supply ADC reference voltage, in case the

			   vref-supply is not added the ADC will use internal voltage

			   reference.

Required Node in the NPCM7xx BMC:

An additional register is present in the NPCM7xx SOC which is

assumed to be in the same device tree, with and marked as

compatible with "nuvoton,npcm750-rst".

Example:

adc: adc@f000c000 {

	compatible = "nuvoton,npcm750-adc";

	reg = <0xf000c000 0x8>;

	interrupts = <GIC_SPI 0 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&clk NPCM7XX_CLK_ADC>;

};

rst: rst@f0801000 {

	compatible = "nuvoton,npcm750-rst", "syscon",

	"simple-mfd";

	reg = <0xf0801000 0x6C>;

};